Four-stroke and two-stroke internal combustion engines, especially automotive internal combustion engines, generally fall into one of two categories, spark ignition engines and compression ignition engines. Traditional spark ignition engines, such as gasoline engines, typically function by introducing a fuel/air mixture into the engine's cylinders, which is then compressed and ignited by a spark plug. Traditional compression ignition engines, such as diesel engines, typically function by introducing or injecting pressurized fuel into the engine's cylinders near top dead center (TDC) of the compression stroke. Both traditional gasoline engine and diesel engine combustion involve premixed or diffusion flames that are controlled by fluid mechanics. Each type of engine has advantages and disadvantages.
More recently, other types of combustion methodologies or concepts have been introduced for internal combustion engines. One of these combustion concepts is known in the art as the homogeneous charge compression ignition (HCCI) engine. HCCI is a distributed, flameless, controlled auto-ignition combustion process that is controlled by oxidation chemistry, rather than by fluid mechanics. Because HCCI is a distributed, kinetically controlled combustion process HCCI engines can operate with a very dilute fuel/air mixture having a relatively low peak combustion temperature, thus forming low levels of NOx emissions. The fuel/air mixture for HCCI combustion is relatively homogeneous, and, therefore, the locally fuel rich zones that form smoke and particulate emissions are eliminated.
At medium engine speed and load, recirculated or trapped exhaust gas may be used to heat the intake charge in order to encourage auto-ignition. This method, however, does not work satisfactorily at or near idle speed and load conditions. At or near idle speed and load, there may be insufficient heat energy within the exhaust to heat the intake air thereby enabling stable auto-ignition. As a result, at the idle condition, the cycle-to-cycle variability of the combustion process is too high to allow stable auto-ignition combustion. Consequently, one of the main difficulties in operating HCCI engine has been to control the combustion process properly such that robust and stable combustion with low emissions, optimal heat release rate, and low noise can be achieved over a range of operating conditions.